The invention relates generally to cardiac pacers, and more particularly to noninvasively programmable implantable cardiac pacers employing microcomputer techniques.
The two major pumping chambers in the heart are the left and right ventricles. Simultaneously contracting, these chambers expel blood into the aorta and the pulmonary artery. Blood enters the ventricles from the left and right atria, respectively. The atria are smaller antechambers which contract in a separate action which precedes the major ventricular contraction by an interval of about 100 milliseconds (ms), known as the AV delay, approximately one-eighth of the cardiac cycle. The contractions arise from a wave of electrical excitation which begins in the right atrium and spreads to the left atrium. The excitation then enters the atrio-ventricular (AV) node which delays its passage via the bundle of His into the ventricles.
Electrical signals corresponding to the contractions appear in the electrocardiogram. A small signal known as the P-wave accompanies atrial contraction while a much larger signal, known as the QRS complex, with a predominant R-wave, signifies ventricular contraction. Ventricular repolarization prior to the next contraction is marked by a small signal in the electrocardiogram known as the T-wave.
The physiology of the heart is known to be compatible with various interactive pacing systems. The P and R waves can be very reliably detected as timing signals by electrical leads in contact with the respective heart chambers and used to synchronize and inhibit stimulation pulses with natural cardiac activity. The typical implanted cardiac pacer operates by supplying missing stimulation pulses on a pacing lead attached to the ventricle. The R-wave can be sensed by the same lead. An additional lead contacts the atrium to sense P-waves, if desired. In AV sequential pacers, discussed below, the atrial lead is also used for atrial stimulation.
One of the problems treated by cardiac pacers is heart block caused by impairment of the ability of the bundle of His to conduct normal excitation from the atrium to the ventricle. It has long been apparent that in treating this form of heart disease it is desirable to base stimulation of the ventricles on the P-wave cycle. This synchronization maintains the heart's normal physiological pacing pattern. Thus, the sino-atrial node, which governs the interval between atrial depolarizations (i.e., the atrial rate) according to the body's needs, controls the artificial ventricular rate in the normal manner.
Patients without normal atrial activity, as in symptomatic bradycardia, often have a need for atrial stimulation as well as ventricular stimulation which alone achieves about seventy-five percent of the combined volume flow. AV sequential pacers have been proposed for stimulating the atria and the ventricles. The system in the 207,003 application, for example, senses and stimulates on both atrial and ventricular leads to provide atrial-based, AV sequential, ventricular-inhibited pacing also referred to as dual demand pacing (DDD).
In the past, pacers offered to the medical community have been implemented by analog or digital timing techniques. Digitally timed pacers with externally programmable pulse parameters have been on the market for several years. For example, the "Omni-Atricor" marketed by the assignee of the present applicaiton, Cordis Corporation, employs a reed switch in the implanted pacer which responds to a pulsating magnetic field produced by a magnetic impulse programmer such as Cordis' programmer 222B to program rate, pulse amplitude and other variables. The reed switch is also used to activate a magnet rate mode when a permanent magnet is placed near the pacer causing it to revert to a fixed rate mode in which it will not respond to natural activity.
Cardiac pacers are life supporting, therapeutic medical devices. They are surgically implanted and remain within a living person's body for years. The vital considerations in cardiac pacing technology tend to dictate a conservative approach, if not reluctance, toward commercially exploiting new developments in electronic circuitry. These tendencies are enhanced by the fact that the relatively simple functional requirements of prior art pacers have been easily implemented using preexisting well-established hardware circuit configurations, and also by the state of the art in compact batteries. The nonrechargeable lithium compound batteries used in most pacers today limit current drain to avoid unnecessary replacements which require surgery and reprogramming of an expensive new pacer. Reliability is the chief concern, however, followed closely by compactness and low current drain.
Digital microcomputers have been suggested for implants before. See, for example, the 207,003 application and U.S. Patent Application Ser. No. 195,665, filed Oct. 9, 1980 by Alan Lesnick, entitled "Implantable Externally Programmable Microprocessor-Controlled tissue Stimulator", assigned to the assignee of the present application. Microcomputers are suited by design to making simple or complex logical decisions and taking alternative action repeatedly in the same manner. Microprocessor technology presents the challenge of making a pacing routine which monitors sense amplifier outputs indicative of spontaneous activity of the heart and safely determines and provides the type of stimulation that would be best suited to the existing condition. It is conceivable that the pacer of tomorrow will diagnose the patient's cardiac function, and prescribe and select the correct stimulation routine for treatment or pacing. The chief problem in exploiting this technology is safety despite increased opportunities for versatility and complexity.
Many patients who need cardiac pacers have a history of cardiac arrhythmia. One of the most feared arrhythmias, particularly for patients wearing physiological pacers, is atrial tachycardia. In this disorder the sinus rate accelerates uncontrollably. Heart rates of 180 to 300 beats per minute are typical. The problem with atrial tachycardia is how to define it electrically so that the implanted microprocessor will be able to recognize it, how to react to it when it happens, how to decide when it is over, and how to safely resume normal pacing.
The atrial arrhythmia response mode disclosed in the 207,003 application embodies some of the key elements of a practical atrial tachycardia response system. Since it is designed to choose and execute a complex treatment automatically, there is ample interest in safeguards designed to assure even greater control of the entry and exit modes for this novel response system.
Ideally, in a microprocessor-based pacer it is desirable to retain the programmability of pacer parameters and to enable the use of preexisting programmers which have already been widely marketed. Programming, however, at least in the approach taken in the 207,003 application, interrupts the pacing routine asynchronously at a random point. The manner of returning to the main pacing routine after such programming is another one of the subjects of the present invention.